This invention relates generally to electronic circuitry and more particularly to electronic circuitry adapted to multiply/divide analog input signals.
As is known in the art, electronic circuitry adapted to multiply/divide analog input signals has a wide varity of applications. One such circuit, or so-called "log-antilog multiplier", includes four transistors having serially coupled base-emitter junctions. An output current is produced in the collector electrode of an output one of such transistors which is, to an approximation, proportional to the product of a pair of currents fed into the collector electrodes of two of the other ones of the transistors divided by a reference current fed into the collector electrode of the fourth one of such transistors. With such arrangement the effective ohmic emitter resistances of the transistors introduce a net error voltage in the circuit, thereby adversely affecting the accuracy of the multiplication/division process. One technique suggested to remove this source of error (discussed in U.S. Pat. No. 3,805,092 issued Apr. 16, 1974) is to provide a compensation resistor connected between the base electrodes of a pair of the transistors. An operational amplifier, coupled to the collector electrode of an output transistor, is also provided to produce an output voltage proportional to the product of the pair of currents divided by the reference current. The voltage is used to produce a compensation current in the compensation resistor to remove the net error voltage produced by the ohmic emitter resistances of the transistors. While such technique may be useful in some applications, the use of an operational amplifier in the output makes the use of such circuit difficult, if not impractical, for integrated circuit fabrication because such amplifier, being fed by the output current, produces thermal gradients across the wafer which have significant adverse effects on the linearity of other devices also formed on such wafer. Further, the use of such operational amplifier in the output as part of the integrated circuit generally limits the application of the integrated circuit to an analog multiplier circuit and therefore such integrated circuit may not be readily adapted for use in other applications, such as: a variable gain amplifier, modulator, demodulator, AGC amplifier, RMS to D.C. converter, divider, square root circuit, etc..